The heart of a computer for longitudinal recording is a magnetic disk drive which includes a rotating magnetic disk, a slider that has a transducer of write and read heads, a suspension arm above the rotating magnetic disk, and an actuator arm that swings the suspension arm to place the transducer of write and read heads over selected circular tracks on the rotating magnetic disk. When the magnetic disk is stationary, the suspension arm biases the slider towards contact with the surface of the magnetic disk. When the magnetic disk rotates, air is swirled by the rotating magnetic disk, causing the slider to ride on an air bearing a slight distance from the surface of the rotating magnetic disk. When the slider rides on the air bearing, the transducer of write and read heads is employed for writing magnetic impressions to and reading magnetic signal fields from the rotating magnetic disk. The transducer of write and read heads is connected to processing circuitry that operates according to a computer program to implement the write and read functions.
A commonly used write head includes first and second write poles, a write gap, a coil, and first, second and third insulator stacks. The write gap, coil and insulator stacks are sandwiched between the first and second write poles. The first and second write poles are connected at the back of the write head. Current conducted to the coil induces a magnetic flux in the first and second write poles which cause a magnetic field to fringe out at the air bearing surface of the write head for the purpose of writing the aforementioned magnetic impressions in circular tracks on the aforementioned rotating magnetic disk.
A commonly used read head includes Ni—Fe first and second shields, Al2O3 first and second read gaps, a giant magnetoresistance (GMR) sensor in a read region, and longitudinal bias stacks in two side regions. The GMR sensor and the longitudinal bias stacks are sandwiched between the first and second read gaps, which are in turn sandwiched between the first and second shields.
In order to perform longitudinal magnetic recording at ultrahigh densities of above 100 Gb/in2, the read head has been progressively miniaturized by reducing its sensor width to as narrow as 60 nm and its gap length to as narrow as 100 nm. Currently, an even narrower width is explored with electron-beam lithography, while an even narrower read-gap length is explored with a thinner GMR sensor inserted into thinner Al2O3 first and second read gaps.
On the other hand, the Ni—Fe first and second shields still remain as thick as more than 1,000 nm. These shields must be thick enough to shield the GMR sensor from unwanted magnetic fluxes stemming from a rotating magnetic medium, while allowing the GMR sensor to only receive confined magnetic fluxes during a read process. To ensure shield efficiency, these shields must exhibit anisotropic soft magnetic properties, such as an easy-axis coercivity (HCE) of below 10 Oe, a hard-axis coercivity (HCH) of below 2 Oe, and an uniaxial anisotropy field (HK) of below 20 Oe. These shields must be also magnetically stable against strong write fields during a write process, in order not to induce noises in the read process. To ensure strong magnetic stability, these shields must exhibit a negative saturation magnetostriction (λS).
For perpendicular magnetic recording recently extensively explored for ever higher densities, a 300 nm thick ferromagnetic film is used as a main write pole, thinner first and second shields are used to minimize thermal extrusion at an air bearing surface, and thinner first and read gaps are used to increase linear densities. Due to this miniaturization of the transducer of the write and read heads, the GMR sensor becomes more susceptible to strong write fields stemming from the nearby main write pole. It thus becomes more stringent for the first and second shields to exhibit strong magnetic stability.